The 1991 geomagnetic jerk as seen at the Earth's surface and the core-mantle boundary

Abstract

SUMMARY We determine the occurrence times of geomagnetic impulses (jerks) around the year 1991 in the three geomagnetic secular variation components for the Earth's surface by a simple optimization algorithm. The geomagnetic field models we use are the low-degree parts of the models CM4 and C3FM. We find that the temporal jerk pattern can be detected in fields (n≤ 4), from which the spherical harmonic degrees n = 2 or n = 3 (tangential) and n = 4 (radial) are representative. To calculate the secular variation components at the core–mantle boundary (CMB) we apply the non-harmonic downward continuation (NHDC) method. For the mantle conductivity, three estimates, dependent on the radius, are assumed with conductances between 107 S and 2 × 108 S. The knowledge of the secular variation components at the CMB allows us to track the global distributions of jerk occurrence times in dependence on the mantle conductivity estimate. We find for each component a typically shaped, global topology for the location of the jerk occurrence times at the Earth's surface and the CMB. For the tangential (ϑ, ϕ)-components, these global topologies show always the well-known temporal bimodality on each surface. Another characteristic feature is found for the jerk of the r-component. It displays a double jerk centred around 1991 consisting of a v-shaped and a reversely v-shaped part, which are significantly correlated. Between the CMB and the Earth's surface, we find time delays in the range of 1–2 yr for the tangential and less than 1 yr for the radial jerk components. To understand these time delays, comparisons at fixed locations are carried out to check the influence of the respective conductivity function. For studying the time delay effects, we apply the inversion set-up of the NHDC to calculate simulated temporal oscillations and derive analytical expressions approximating the phase shifts. We find that jerk occurrence time delays and simulated phase shifts of temporal oscillations have a similar behaviour with respect to the influence of the conductivity and for the radial and the tangential components, respectively. In addition, a new concept for determining a jerk amplitude is presented briefly. This so-called dynamical jerk morphology, which forms a portion of the geomagnetic secular acceleration, is defined for each component by a time function on the considered surface. Its temporal motion patterns at the CMB are likely related with jerk originating processes in the fluid outer core.